1. Field of the Invention
This invention relates generally to dividing circuits and, more particularly, is directed to a dividing circuit for use in an AM stereophonic broadcast system.
2. Description of the Prior Art
Systems for transmitting and receiving AM stereo signals are known in the art. In one such system, described in U.S. Pat. No. 4,218,586, an AM stereo broadcast system is disclosed which is compatible with both monaural and stereo receivers. In such system, a rectangular modulated signal is multiplied with a distortion correcting signal cos .phi., where EQU .phi.=tan.sup.-1 L-R/l+L+R
Accordingly, a composite stereo broadcast signal is produced, as follows: EQU V(t)=cos .phi.{(l+L+R) cos .omega.t+(L-R) sin .omega.t}
This equation may be rewritten as follows: EQU V(t)=(l+L+R) cos (.omega.t+.phi.)
This signal, which has been non-linearly modified by the distortion correcting signal cos .phi., is then transmitted and is compatible with both a monophonic receiver and a stereo receiver. When the transmitted signal is received by a monophonic receiver, it is demodulated by an envelope detector and an output signal proportional to (L+R) is produced. When the transmitted signal is received by a stereo receiver, in order to prevent distortion upon demodulation of the signal, the distortion correcting signal cos .phi. is detected and the received stereo signal V(t) is divided by the distortion correcting signal cos .phi. to produce the original rectangular modulated signal. In this regard, it is to be appreciated that the AM stereo receiver requires a dividing circuit for cancelling or eliminating the aforesaid distortion correcting signal cos .cent..
The dividing circuit is generally constructed of a differential amplifier comprised of two transistors, one of the transistors being supplied with the stereo signal V(t) at its base, with the bases of the two transistors being connected to each other by two oppositely-poled series-connected diodes. A signal corresponding to the distortion correcting signal is supplied to the connection point between the oppositely-poled diodes. In this manner, the original stereo output signal is produced at the collectors of the two transistors.
In order to prevent distortion in the stereo output signal from becoming large, the value of the resistances provided at the bases of the two transistors of the dividing circuit are set much greater than the operating resistances of the diodes. However, in the case where the dividing circuit is operated in its full dynamic range such that the current flowing through the diodes is cut off, that is, at the limits of the dynamic range thereof, the operating resistances of the diodes become large and the aforementioned condition of the input base resistances being much greater than the operating resistances of the diodes is not satisfied. As a result, distortion in the output stereo signal becomes great. It should therefore be appreciated that such dividing circuit cannot be used with a wide dynamic range.
Further, because of the condition that the input resistances at the bases of the transistors are much greater than the operating resistances of the diodes, a large in-phase mode signal of the current flowing through the diodes results. In such case, if the balance between the characteristics of the transistors, the diodes and the input base resistances to the transistors is poor, the signal corresponding to the distortion correcting signal, which is supplied to the dividing circuit, is undesirably mixed with the output stereo signal from the differential amplifier which, in turn, results in further distortion.